1. Field of the Invention
The present invention relates generally to blower assemblies for use in respiratory treatment of patients. More specifically, the present invention relates to a quiet blower apparatus and a system and method for reducing blower noise.
2. Description of Related Art
Blower assemblies have conventionally been used in a variety of situations. In many of these situations, it is desirable for the blower to be quiet.
One such situation where a quiet blower is important is during the treatment of Obstructive Sleep Apnea (OSA) using Continuous Positive Airway Pressure (CPAP). CPAP technology was invented by Sullivan and is described in U.S. Pat. No. 4,944,310. During OSA treatment with CPAP, a supply of air or breathable gas at a positive pressure is created by a blower assembly and is delivered by an air delivery conduit to a patient interface, such as a nasal mask. Since this method of treatment is generally administered while a patient is sleeping, it is important that the blower operates quietly so as not to disturb the patient.
Generally, the blower is a dynamic pump that includes an electric motor, an impeller, and a volute. The volute is a housing that surrounds the impeller and has an inlet and an outlet. The electric motor spins the impeller which imparts rotational motion to the air. A pressure develops inside the volute as a result of a restriction at the outlet of the volute.
Treatment of OSA using CPAP has been successful but reduction of noise during treatment has been a continuing challenge. Such reduced noise is also desirable in other applications of blowers such as air-conditioning units, hydraulic pumps and other such apparatus. Some of the noise produced by the blower can be reduced by mechanically isolating the blower. Specifically, by mounting the blower on springs or a rubber block inside a heavy and stiff enclosure, some of the blower noise can by mechanically isolated. However, the prior art methods have only been marginally successful and they make the device (such as the CPAP device) large and heavy when, in fact, portability and a smaller size is often desirable.
Other applications where the present invention can be useful is in computers and other apparatus where a blower is used to cool electrical circuits as a lot of these apparatus are used in quiet places. Other possible applications include other types of medical devices and the like.
Methods of blower noise reduction using mufflers and other in-line filter mechanisms have also been tried. However, small mufflers and in line filters are generally not effective at reducing relatively low frequency noise and require a certain size to accomplish better noise reduction. In contrast, the membrane system is not primarily an in-line filter but is a mechanism for providing noise reduction by interference of the inlet and outlet noise. Thus, the size of the interfering system is not restricted by resonance effects linked to a filter design. It will be beneficial to use this membrane system in conjunction with other filter elements, particularly where the filter elements are designed to reduce high frequency noise beyond the frequencies where the membrane system is effective. It will be seen that provided the acoustic path lengths from the inlet and outlet are similar, means for high frequency filtering may be included in both the inlet and outlet path prior to effecting noise cancellation at relatively low frequencies with the membrane.